Which sentence uses an action verb?

a.
seven horses were in the pasture.

b.
we bought some vegetables for our salad.

c.
are you ready for the big game?

d.
tomorrow will be the start of spring.

An action verb refers to an action, that is not a state. Verbs which are NOT action verbs are all verbs of states, most typically the verb "to be". So sentences with the verb "to be" are likely not to contain action verbs: here they're options A C and D - those don't use an action verb.

The correct answer is B. We bought some vegetables for our salad.

and the action verb is: "bought".